1. Field of the Invention
This invention relates to a method for producing a multilayer printed wiring board and more particularly to a method for producing a multilayer printed wiring board provided with via holes (hereinafter referred to as "via T/H's") for electrically connecting circuit patterns of surface layers to circuit patterns of inner layers.
2. Description of the Prior Art
With rapid advance in electronic equipments, the development of electronic components, typified by semiconductor components, of high packaging density, high function and miniaturization has recently been advanced with great strides. In addition, the development of printed wiring boards for packaging these electronic components has recently been required for an increase in wiring channels thereof.
In general, if many wiring channels are desired, a multilayer printed wiring board having multilayered wiring layers is used. Particularly, since the multilayer printed wiring board provided with the via T/H's for selectively connecting the circuit patterns of the first- and second-surface layers to the circuit patterns of inner layers can dramatically increase the number of the wiring channels, such multilayer printed wiring board has widely been used in recent years.
Referring now to FIGS. 1 to 5, which are schematically and partially cross-sectional diagrams for explaining, in order, steps of a method for producing a multilayer printed wiring board of the prior art, a completed multilayer printed wiring board has a structure provided with via T/H's as shown in FIG. 5 and can be produced in the following manner.
First, as shown in FIG. 1, connecting pads 2 and inner circuit patterns 3 are formed by patterning upper and lower faces of a copper-clad insulating substrate (an inner wiring substrate) 1 to obtain prescribed patterns.
Then, as shown in FIG. 2, copper foils 5a and 5b for surface layer are disposed on the both faces of the substrate 1 through prepreg sheets (a glass cloth impregnated with an adhesive and dried) 4a and 4b, respectively, to assemble these members, and heat and pressure are applied to the resulting assembly using a pressure device. Thus, the copper foils 5a and 5b are integrally bonded to the substrate 1 by sandwiching each of the prepreg sheets 4a and 4b between each of the copper foils and each face of the substrate.
Next, as shown in FIG. 3, half through-holes 6 are selectively bored in the copper foils 5a and 5b and the prepreg sheets 4a and 4b so that they reach from the surface of each of the copper foils 5a and 5b to the connecting pads 2. Also, a through-hole 7 is selectively bored through the assembly so that it runs through one surface of the copper foils 5a and 5b to the other surface.
Thereafter, as shown in FIG. 4, copper plating is applied to the whole surface of the assembly including the inner walls of the half through-holes 6 and the through-hole 7 according to the conventional method to thus form a copper plating layer 8. By forming the copper plating layer 8 as mentioned above, the connecting pads 2 are electrically connected to the copper foils 5a and 5b. Since the copper plating layer 8 is also deposited on the inner walls of the through-hole 7, a through hole (i.e. a copper plated-through hole) 9 which electrically connects the copper foil 5a to the copper foil 5b is formed running through the assembly.
Finally, the copper plating layer 8 and the copper foils 5a and 5b are patterned to thereby give a completed multilayer printed wiring board provided with the via T/H's 10, the through hole 9 and wiring circuit patterns 11, as shown in FIG. 5.
Referring now to FIGS. 6 to 11, which are schematically and partially cross-sectional diagrams for explaining, in order, steps of another method for producing a multilayer printed wiring board of the prior art, another completed multilayer printed wiring board, which has a structure provided with via T/H's as shown in FIG. 11, can be produced in the following manner.
First, as shown in FIG. 6, through-holes 22 are selectively bored through a copper-clad insulating substrate 21 whose upper and lower faces are provided with copper foils 23a and 23b, respectively.
Then, as shown in FlG. 7, copper plating is applied to the whole surface of the substrate 21 including the inner walls of the through-holes 22 to thus form a copper plating layer 24. In this case, the copper plating layer 24 is deposited on the inner walls of the through-holes 22 and thus through holes (i.e. copper plated-through holes) 25 which electrically connect the copper foil 23a to the copper foil 23b are formed running through the substrate 21.
Next, as shown in FIG. 8, the copper plating layer 24 and copper foil 23b of one face of the substrate 21 are patterned to give inner circuit patterns 26.
In the same manner as mentioned above, there are prepared two of the substrates 21, one face of each substrate having the circuit patterns 26 formed thereon and the other face being covered by the copper foil and copper plating layer.
Then, as shown in FIG. 9, the two substrates 21 are disposed so that the faces having the inner circuit patterns 26 face each other and a prepreg sheet 27 is sandwiched between the faces, to assemble these members, and heat and pressure are applied to the resulting assembly or structure using a pressure device. Thus, the two substrates 21 are integrally bonded to each other through the prepreg sheet 27 sandwiched therebetween.
Next, as shown in FIG. 10, after a through-hole (not shown) is bored at desired position so that it runs through the first-surface of the assembly to the second-surface thereof, another copper plating is applied to the whole surface of the assembly including the inner wall of the through-hole to thus form a copper plating layer 28. The copper plating layer 28 is deposited within the through holes 25 to give via T/H's 29, and also the copper plating layer 28 is deposited on the inner wall of the above through-hole (not shown) to thus give a through hole (i.e. a copper plated-through hole) 30 which electrically connects the copper plating layer 24 of one face of the assembly to the copper plating layer 24 of the other face thereof.
Finally, the copper plating layers 28 and 24 and the copper foils 23a are patterned, to thereby give the completed multilayer printed wiring board provided with the via T/H's 29, through hole 30 and wiring circuit patterns 31 as shown in FIG. 11.
With the above-mentioned conventional method for producing the multilayer printed wiring board, however, there are some problems as discussed below.
In the method as shown in FIGS. 1 to 5, when the half through-hole 6 is drilled in the copper foil and prepreg sheet, it is necessary to set the extremely slow drilling speed since it is difficult to control a drilling depth. For this reason, in this conventional method, the production cost for the multilayer printed wiring board is very high and also the production term becomes longer. In addition, since a diameter of the half through-hole 6 is small, for instance, being of the order of 0.1 to 0.4 mm, the drill is easily broken during drilling and thus drilling failure easily occurs. Moreover, when the connecting pads having a width of 0.15 to 0.3 mm stand close together in a pitch of 0.2 to 0.5 mm, the drill easily slips from the required drilling position of the half through-hole 6.
Furthermore, the method as shown FIGS. 6 to 11 requires a plurality of drilling processes and plating processes and is troublesome, and thus the production term becomes longer. In addition, it is required to apply a copper plating onto the thin substrate 21, and in such a case failure easily takes place due to handling miss in the plating process. Thus, problems exist in that the payroll increases and also the production yield ends in an unsatisfactory result.